The present invention relates to a method for producing a thin-film magnetic recording medium, and more particularly relates to a method for producing a thin-film magnetic recording medium having an excellent corrosion resistance.
Conventionally, to produce a widely used coating-type magnetic recording medium, a nonmagnetic base is coated with a powdered magnetic material dispersed in an organic binder such as a copolymer of vinyl chloride and vinyl acetate, a copolymer of styrene and butadiene, epoxy resin, polyurethane resin, or the like, and the coating is then dried. The powdered magnetic material is selected from oxide magnetic powder, for example, .gamma.-Fe.sub.2 O.sub.3, Fe.sub.2 O.sub.3 , Co-doped .gamma.-Fe.sub.2 O.sub.3, Co-doped Fe.sub.3 O.sub.4, a bertholide composed of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, Co-doped bertholide, CrO.sub.2, or the like, and alloy magnetic powder containing Fe, Co, Ni, etc., as principal components.
Recently, to obtain higher recording densities, a ferromagnetic thin film formed by vacuum evaporation, sputtering, ion plating, or the like, has received much attention as a so-called thin-film magnetic recording medium using no binder, and effort has been made to achieve practical applications of the ferromagnetic thin film.
The conventional coating-type magnetic recording medium, however, is unsuitable as a high-output high-density recording medium because it uses a metal oxide having a small saturation magnetization as its primary magnetic material, and the volume content of the magnetic material in the magnetic layer is only about 30 to 50%. Further, the conventional coating-type magnetic recording medium has a drawback in that it requires a complicated manufacturing process, which further requires large-scale incidental facilities for solvent recovery and pollution control. The metal thin-film magnetic recording medium, however, has an advantage in that ferromagnetic metal having saturation magnetization larger than that of oxide can be formed as an exceedingly thin film without using any nonmagnetic material such as an organic binder or the like. A recording/reproducing magnetic head having a gap length of 1.0 .mu.m or less has been used to achieve higher density recording. With such a head, however, the recording depth in the magnetic recording layer is shallow, and therefore the entire thickness of the magnetic layer can be utilized for recording. As a result, this thin-film magnetic recording medium is extremely excellent for high-output high-density recording.
Of the available methods for producing a thin-film magnetic recording medium, methods using vacuum evaporation for film formation have the advantages that the film forming speed is high, the film-formation process is simple, the drying process does not require waste liquid disposal, etc. Of the evaporation methods, the oblique-incidence vacuum evaporation method in which a vapor flow of a magnetic material is made to be obliquely incident on a nonmagnetic base is superior in practical use because the method is relatively simple to implement and at the same time a film having excellent magnetic characteristics can be obtained.
The thin-film magnetic recording medium, however, has a drawback in that characteristics for practical use such as corrosion resistance, durability, and the like, are inferior to those of the above-discussed coating-type magnetic recording medium. There have been proposed various methods for eliminating this drawback. In one of those methods, a gas is fed into a vacuum tank while a magnetic material is evaporated in the vacuum tank so that the gas and the magnetic material are caused to react with each other to form a thin-film magnetic layer. (See Japanese Unexamined Pat. Publications Nos. 58-41442 and 58-41443.) For example, compared with the case where film formation is performed without supplying any reactive gas into the vacuum tank, the durability and corrosion resistance are exceedingly improved in the case where an alloy of Co (80%) and Ni (20%) is used as the magnetic material and film formation is performed while oxygen gas is supplied into the vacuum tank. Further, the present applicants have proposed a way of improving the corrosion resistance by supplying oxygen gas into the vacuum tank. (See Japanese Unexamined Pat. Publication No. 62-121929.) Moreover, for a combination of gases which cannot be made to react only by supplying a gas into a vacuum tank, the problem can be solved by ionizing and exciting the gas so as to make its reaction activity high. To this end, for example, there has been proposed a method in which Fe is used as a magnetic material and nitrogen gas, which is a reactive gas, is ionized to thereby form an iron nitride magnetic layer. (See Japanese Unexamined Pat. Publication No. 60-231924.)
In the foregoing method, although considerable improvements are achieved over the conventional approach, the corrosion resistance of the obtained thin-film magnetic layer has sometimes been insufficient when the formation of the thin-film magnetic layer is performed at a high speed of 200 .ANG./sec or more.